In terms of their capacity to decompose a casein substrate, the activity of nuclear proteinases was studied in the leukocytes of the rabbit blood taken before and after their joint gamma-irradiation. There was a substantial increase in the activity of proteinases in the leukocytic nuclei 2 and 48 hours after animals had been irradiated in a dose of 5 Gy as compared with those obtained prior to irradiation. Ninety six hours after irradiation the activity showed an average decrease to 40-60% of the control level. There was a correlation between the proteolytic activity and the irradiation dose: there were average 1.75- and 3.25-fold activity increases with 2.5 and 5.0 Gy, respectively. The findings provide a rationale for examining the proteinase activity of human peripheral leukocytic nuclei to clinically estimate radiation exposures.